Today, laser diodes have been put into practical use as light sources for optical communications. Laser diodes used in optical communications are designed to lase wavelengths of 1.33 .mu.m and 1.55 .mu.m which are lost only a little in quartz fibers. Also, laser diode lasing wavelengths of approximately 780 nm have been extensively used in digital audio applications, laser printers, and other applications. On the other hand, infrared light having wavelengths longer than 780 nm is not visible and so laser elements emitting shorter wavelengths have been developed to make the laser light usable in the visible range. For example, compact visible laser light sources are required for high-density optical memories and displays. However, the wavelengths of laser diodes which have been put into practical use today are approximately 650 nm, i.e., red light. Laser diodes emitting shorter wavelengths of laser light, e.g., green light and blue light, have not been put into practical use. In this connection, attempts have been made to obtain shorter wavelengths of laser light by doubling the frequency by the use of second-harmonic generation. If this method is utilized, infrared laser light, for example, having a wavelength of 850 nm can be converted into blue light having a wavelength of 425 nm. Unfortunately, the difficulties of temperature control and cost prevent production of a compact SHG laser.
Accordingly, an up-conversion laser for converting infrared diode laser light into blue or green laser light, by making use of an up-conversion phenomenon, is expected to be developed. It is reported that as a light-emitting device of this kind, up-conversion lasers have been successfully oscillated, using a single crystal of a fluoride such as YLiF.sub.4 :Er or heavy metal fluoride glass as typified by ZBLAN glass (Wilfried Lenth et al., Optics & Photonics News, 3, [3 ], 8-15 (1992)). However, the oscillation intensity is insufficient and far from practicability. To have laser oscillation of sufficient intensity, a material having a better light conversion efficiency is required. To satisfy this requirement, various glass materials have been developed, as disclosed in Japanese Patent Laid-Open Nos. 295828/1991, 12035/1992, and 328191/1992.
However, with these glass materials, it is difficult to enhance the concentration of rare-earth ions such as Er.sup.3+ which is a light emission lasing source. Therefore, it is difficult to obtain a compact device having a short optical length. It is known that even if the rare-earth concentration is higher, the light emission efficiency deteriorates because of a phenomenon known as cross relaxation. Furthermore, these glass materials are generally multi-component materials and so it is technically difficult to fabricate a light-emitting substance of uniform composition.
The above-described glass materials are fluorides or oxides. It has been pointed out that chlorides produce higher up-conversion emission efficiencies than these glass materials (Tabe et al., Ceramics, 26 (1991), 144). But today, any transparent light-emitting substance which consists of chlorides, has a sufficient up converson efficiency, and can be used as up conversion laser devices, is not available.